The present invention relates to in situ liquefaction and hydrogenation of coal and, more particularly, to a method of solution mining an underground coal seam involving the heating, pressurizing and chemical processing of the coal so that it may be extracted from underground as a liquid.
Geological exploration has demonstrated the existence of countless relatively thick coal seams at depths of on the order of 500 meters. Heretofore, the depth of burial has hindered recovery of the coal because of the high cost of strip mining or conventional mining at such depth. Furthermore, the recovery of the coal in many such seams has been further complicated because the coal is interspersed with layers of shale which make the coal uneconomical to mine with continuous mining equipment, principally because of rapid wear caused by the shale. However, because of the present uncertainty about the availability of known liquid petroleum resources, current and predicted prices of crude oil, and rapid depletion of the world's oil, the successful, efficient extraction of deep deposits of coal is of significant potential commercial importance.
Coal, in general, is a solidified hydrocarbon. Although anthracite coal is close enough to pure carbon to be considered insoluble, the more abundant bituminous coals, which have a molecular hydrogen to carbon ratio of approximately 0.8 hydrogen to one carbon and a chemical structure bound to a significant extent by oxygen bonds between multiple benzene ring-type hydrocarbons, may be dissolved in other benzene ring compounds at high temperature because of the large hydrogen content.
Since bituminous coal is soluble in appropriate solvents, above-ground hydrogenation processes heretofore proposed and utilized for producing petroleum from coal depend upon the initial solubility of coal at high temperature and high pressure in an appropriate solvent. In those hydrogenation processes, the coal is hydrogenated by donor hydrogen from the solvent which is then reconstituted or hydrogenated in either a separate or the same process so that the solvent is sequentially used as a donor and then recipient of hydrogen. The hydrogenated coal becomes a liquid composed of hydrogen-poor solvents. However, the above-ground hydrogenation of coal requires the mining of the coal, processing of the coal and expensive reactor or pressure vessel for providing the high temperature and high pressure required to induce the hydrogen exchange between the solvent and the coal.
Inasmuch as the reactions involved in the hydrogenation of coal also proceed quite rapidly for underground processing, provided the hydrogen or hydrogen donor and the requisite temperature and pressure are available, it has been suggested that underground coal may be removed through a drill pipe by a process similar to the Frasch process which is used for extracting sulphur from deep deposits. The Frasch process utilizes hot water which is pumped down a pipe in a well bore to melt the sulphur; the liquefied sulphur is forced up to the surface through another pipe.
Although there are a few high boiling, aromatic solvents, e.g., phenanthrene and carbozole, having a relatively high molecular weight and a capability of dissolving coal at atmospheric pressure when heated to an appropriate solubility temperature, the fraction of those solvents in the coal tars is too small for commercial utilization in cyclic extraction processes. Accordingly, low boiling, benzene ring compounds having a comparatively low molecular weight should be used. Since these low boiling, aromatic compounds have higher vapor pressures at the temperatures required for solubility, their employment in the liquefaction and extraction of coal, on a commercial scale, requires high pressure as well as high temperature. In addition, hydrogen gas should be added to the solvent, requiring a high pressure for a finite solubility.
Since the liquefied coal being extracted can be used to transfer its heat to the down-flowing stream of hydrogenated solvent, the underground heat required to liquefy the coal is dependent upon the amount of heat diffused into the rocks surrounding the coal seam. Calculations indicate that the diffusion of heat into the surrounding rock media approximately doubles the heat required for hydrogenation. Moreover, the diffusion of the solvent is increased by the relatively high pressures utilized in the hydrogenation process which cause the solvent to leak into the strata above and below the coal seam. Thus, although the possibility of in situ hydrogenation of coal has been long recognized, a commercially feasible method for hydrogenating coal in situ has heretofore been impossible because of inadequate strength in the strata above and below the coal seam to permit sufficient pressure to be developed in the coal seam for effective hydrogenation of the coal.